Four cases of mysterious illness in Northern Virginia horses have been attributed to equine herpesvirus (EHV) myeloencephalopathy, a rare neurologic version of EHV type 1, which is typically recognized in its respiratory form as rhinopneumonitis. Three horses at Fox Chase Farm in Middleburg have been euthanized, and one pony is recovering at the farm under supportive care. The incidents have area horse owners concerned about travel and exposure of their horses, and veterinarians are alert for any other unusual symptoms.

Joseph P. Garvin, DVM, is laboratory director of the Virginia Department of Agriculture and Consumer Services’ Warrenton Regional Animal Health Laboratory, where the autopsies of the second and third horses were performed. While test results at Garvin’s laboratory and the National Veterinary Services Laboratory in Ames, Iowa, have been “inconclusive,” scientists have ruled out other possible neurologic diseases and have strong evidence from clinical signs and histopathology reports that EHV is what caused the disease. “In this instance, we do have negative tests for West Nile virus, equine encephalitis, and for EPM (equine protozoal myeloencephalitis),” said Garvin. “There are still tests pending, and I have my fingers crossed that we’ll have definitive answers. The results have been very suggestive of equine herpesvirus myeloencephalopathy.”

Garvin explained that tests of the tissue are conclusive only if EHV-1 is grown out from the tissue or its DNA is found through one of many very sensitive laboratory tests. To complicate the matter, the virus might not be present in the horses by the time of their deaths, so tests might come back negative.

“The organism causing equine herpesvirus-1 can cause three different forms of the disease,” he explained, “rhinopneumonitis, a respiratory disease of mostly young horses, abortions in pregnant mares, and this neurologic disease.” There are at least seven other strains of equine herpesviruses, named in order of their discovery. (See Article #32 at www.TheHorse.com for more on herpesviruses